Reloading Basics: Neck Tension, Expander Balls, and Bushings

by Sierra Bullets Ballistic Technician Paul Box
One thing that plays a major role in building an accuracy load is neck tension. I think a lot of reloaders pretty much take this for granted and don’t give that enough thought.

So, how much neck tension is enough?

Thru the years and shooting both a wide variety of calibers and burn rates of powder, I’ve had the best accuracy overall with .002″ of neck tension. Naturally you will run into a rifle now and then that will do its best with something different like .001″ or even .003″, but .002″ has worked very well for me. So how do we control the neck tension? Let’s take a look at that.

First of all, if you’re running a standard sizing die with an expander ball, just pull your decapping rod assembly out of your die and measure the expander ball. What I prefer is to have an expander ball that [can give] you .002″ in neck tension [meaning the inside neck diameter is about .002″ smaller than the bullet diameter after passing the expander through]. If you want to take the expander ball down in diameter, just chuck up your decapping rod assembly in a drill and turn it down with some emery cloth. When you have the diameter you need, polish it with three ought or four ought steel wool. This will give it a mirror finish and less drag coming through your case neck after sizing.

Tips for Dies With Interchangeable Neck Bushings
If you’re using a bushing die, I measure across the neck of eight or ten loaded rounds, then take an average on these and go .003″ under that measurement. There are other methods to determine bushing size, but this system has worked well for me.

Proper Annealing Can Deliver More Uniform Neck Tension
Another thing I want to mention is annealing. When brass is the correct softness, it will take a “set” coming out of the sizing die far better than brass that has become to hard. When brass has been work hardened to a point, it will be more springy when it comes out of a sizing die and neck tension will vary. Have you ever noticed how some bullets seated harder than others? That is why.

Paying closer attention to neck tension will give you both better accuracy and more consistent groups.

What about the condition of the inside of the neck? I promise that those cases you just fished out of the ultrasonic cleaner are going to be a lot more grabby than fired cases when you seat bullets. Does that count as more neck tension? You can certainly feel a difference if you use an arbor press to seat bullets, and if you have the K&M with the force measurement option you can see it too.

This article is exactly correct for normal to match loading ammunition. I have loaded semi-auto ammo for 35 years and have always used .003 thousandths neck tension and have never crimped and never had an issue. M1A three position match shooting and AR 308.

Is “springy” the opposite of hard? I’m new to reloading. (I haven’t started, yet, but I’m trying to do research to determine an initial set of tools.) It seems to me that as brass becomes harder, the diameter prior to seating the bullet would be a less reliable indicator of the neck tension. The corollary to that would be that if you are annealing brass prior to sizing(? Or is it shaping?) you would need a different diameter ball than if you are not annealing to get the same diameter prior to seating the bullets. Am I understanding this? Is it the case that the “ball” is not a sphere? Any other urls of strings regarding the basics of reloading would be appreciated!